In the case of large aircraft with a heavy take-off weight there is typically a clear functional separation of the wing and the payload cabin (fuselage). At the end of the fuselage, as far as possible at a large distance from the wing, control surfaces (tail sections) are provided which control the aircraft on its vertical axis and its transverse axis. In this arrangement the elevator unit among other things also has the task of initiating aircraft rotation during take-off, i.e. during the take-off roll procedure to cause rotation of the aircraft on its transverse axis, as a result of which rotation the wing is set at an angle and thus the lift for lift-off is increased.
However, in futuristic projects relating to aircraft, in which in the sense of a so-called “wing-only type design” the payload volume is predominantly installed in the wing region, a configuration results with relatively short lever arms to the elevator control surfaces and rudder control surfaces, in relation to the position of the centre of gravity of mass or the centre of gravity of surface. As a consequence of this, with the same size area, in such an aircraft design the torque of the elevator control surfaces, which torque causes take-off rotation, is less than is the case in a conventional design.
For a large aircraft, in which for reasons of the very heavy maximum take-off weight on each side at least two multi-axis main landing gear units are arranged one behind the other, initiation of take-off rotation would require a disproportionately large and thus heavy elevator control surface, whose oversize would not be required for the actual flight manoeuvres.
In the case of aircraft that feature multi-axis main landing gear units or that, for example as a result of the size and position of the engines, feature particularly long landing gear legs, from the state of the art devices are known which are used to lower or shorten the landing gear legs before or during the retraction procedure. In this way improved loadability and/or improved stowability in the landing gear bay are/is to be provided. Such solutions are known from the Concorde or the AIRBUS A340.
Conversely, it is also known in the case of main landing gear units which for reasons of weight or for reasons of retraction geometry comprise relatively short landing gear legs, by means of special provisions to rotate the landing gear undercarriage in such a way that during take-off and landing at least the wheels of the rearmost axes are guided closer to the ground than the other wheels so as to allow adequate rotation angles (so-called rocking undercarriage).
Furthermore, hydraulic balance systems for landing gear units of aircraft, e.g. Boeing 747, are known by means of which systems, according to the principle of communicating pipes, during rotation that is initiated by the elevator unit, hydraulic pressure equalisation between the more heavily loaded and the less heavily loaded landing gear legs is automatically brought about. However, these systems operate reactively rather than being actively controlled for initiating take-off rotation.
Moreover, from the literature, devices may be known for aircraft with multi-axis main landing gear units, which devices are designed to overcome the difficulties in relation to take-off rotation in that in the initial phase of the take-off roll procedure, up to a defined rate of roll, lift-generating surfaces such as trailing edge flaps are deliberately not employed, and/or in that even lift-spoiling surfaces (spoilers) are extended until a defined lift-off speed is reached, at which point conversely as soon as practicable both the high-lift devices are extended and the spoilers are retracted. In this way fast generation of strong lift is initiated, which is however not very comfortable for passengers, which lift is adequate to take off more or less vertically until the elevator control surfaces are aerodynamically sufficiently effective to manage further rotation (direct lift control). Apart from the lift behaviour that is uncomfortable for passengers, this take-off procedure is associated with disadvantages of high resistance and thus high fuel consumption.